1. Field of the Invention
Embodiments of this invention are generally related to safety valves. More particularly, embodiments of this invention pertain to subsurface safety valves configured to control fluid flow through a production tubing string.
2. Description of the Related Art
Surface-controlled, subsurface safety valves (SCSSVs) are commonly used to shut in oil and gas wells. Such SCSSVs are typically fitted into a production tubing in a hydrocarbon producing well, and operate to block the flow of formation fluid upwardly through the production tubing should a failure or hazardous condition occur at the well surface.
SCSSVs are typically configured as rigidly connected to the production tubing (tubing retrievable), or may be installed and retrieved by wireline, without disturbing the production tubing (wireline retrievable). During normal production, the subsurface safety valve is maintained in an open position by the application of hydraulic fluid pressure transmitted to an actuating mechanism. The hydraulic pressure is commonly supplied to the SCSSV through a control line which resides within the annulus between the production tubing and a well casing. The SCSSV provides automatic shutoff of production flow in response to one or more well safety conditions that can be sensed and/or indicated at the surface. Examples of such conditions include a fire on the platform, a high/low flow line pressure condition, a high/low flow line temperature condition, and operator override. These and other conditions produce a loss of hydraulic pressure in the control line, thereby causing the flapper to close so as to block the flow of production fluids up the tubing.
Most surface controlled subsurface safety valves are “normally closed” valves, i.e., the valves utilize a flapper type closure mechanism biased in its closed position. In many commercially available valve systems, the bias is overcome by longitudinal movement of a hydraulic actuator. In some cases the actuator of the SCSSV includes a concentric annular piston. Most commonly, the actuator includes a small diameter rod piston, located in a housing wall of the SCSSV.
During well production, the flapper is maintained in the open position by a flow tube down hole to the actuator. From a reservoir, a pump at the surface delivers regulated hydraulic fluid under pressure to the actuator through a control conduit, or control line. Hydraulic fluid is pumped into a variable volume pressure chamber (or cylinder) and acts against a seal area on the piston. The piston, in turn, acts against the flow tube to selectively open the flapper member in the valve. Any loss of hydraulic pressure in the control line causes the piston and actuated flow tube to retract, which causes the SCSSV to return to its normally closed position by a return means. The return means serves as the biasing member, and typically defines a powerful spring and/or gas charge. The flapper is then rotated about a hinge pin to the valve closed position by the return means, i.e., a torsion spring, and in response to upwardly flowing formation fluid.
In recent completion techniques, an SCSSV may be run with the production tubing into the hole prior to a cementing operation. Once the cement is cured, the desired formations are perforated through the tubing. Using this technique, however, exposes the SCSSV to the cement during the cementing operation, which may cause the SCSSV to fail prematurely.
Therefore, a need exists for an apparatus and method for protecting the SCSSV from cement infiltrating the SCSSV during the cementing operation.